Subgrades and
Subbases for
Concrete Pavements
Subgrades and Subbases for Concrete Pavements
This publication is intended SOLELY for use by PROFESSIONAL PERSONNEL who are competent to evaluate the significance and limitations of the information provided herein, and who will accept total responsibility for the application of this information. The American Concrete Pavement Asso-ciation DISCLAIMS any and all RESPONSIBILITY and LIABILITY for the accuracy of and the application of the information contained in this publication to the full extent permitted by law.
American Concrete Pavement Association 5420 Old Orchard Rd., Suite A100
A MERI C AN CONCRETE P A V EMENT ASSOCI 9 0 0 0 0
Subgrades and
Subbases for Concrete
Pavements
ACPA is the premier national association representing concrete pavement contractors, cement companies, equipment and materials manufacturers and suppliers. We are organized to address common needs, solve other problems, and accomplish goals related to research, promotion, and advancing best
American Concrete Pavement Association 5420 Old Orchard Rd., Suite A100 Skokie, IL 60077-1059
(847) 966-ACPA www.pavement.com
hauling, cut-fill transition, daylighted, drainable subbase, econocrete, edge drainage, expansive soil, faulting, free-draining, frost heave, frost-susceptible soil, granular subbase, lean concrete, pavement structure, permeable subbase, pumping, reclaimed asphalt pavement (RAP), recycled concrete, separator, sieve analysis, soil, soil classification system, spring subgrade softening, stabilized subbase, subbase, subgrade, unstabilized subbase, volume stability.
Abstract:This engineering bulletin provides the necessary background information for the proper selection and application of subbases and the appropriate consideration of subgrade variables for concrete pavements used for streets, roads and highways. It emphasizes the major objective of obtaining long-lasting uniform support for a concrete pavement. Subgrade soil material classifica-tion and problems (expansion, heaving, etc.) are discussed in detail along with many various opclassifica-tions for stabilized and unstabilized subbases, and the materials that they are composed of. Multiple reasons for avoiding the use of permeable subbases (a.k.a. drainable or open-graded subbases) also are presented.
© 2007 American Concrete Pavement Association All rights reserved. No part of this book may be reproduced in any form without permission in writing from the publisher, except by a reviewer who wishes to quote brief passages in a review written for inclusion in a magazine or newspaper.
ISBN 978-0-9800251-0-1
This publication is intended SOLELY for use by PROFES-SIONAL PERSONNEL who are competent to evaluate the significance and limitations of the information provided herein, and who will accept total responsibility for the appli-cation of this information. The American Concrete Pavement Association DISCLAIMS any and all RESPONSIBILITY and LIABILITY for the accuracy of and the application of the information contained in this publication to the full extent permitted by law.
Executive Summary
. . . viiExecutive Overview
. . . ixChapter 1 – Introduction and Terminology
. . . 1Chapter 2 – Design Principles
. . . 3Uniform Support. . . 4
Influence of Foundation Strength on Concrete Pavement Thickness . . . 6
Influence of Foundation Stiffness on Stresses and Strains in Concrete Pavement Slabs. . . 7
Pavement System Drainage . . . 8
Sources of Moisture in a Pavement Structure . . . 8
Free-Draining Subbases . . . 9
Edge Drainage Systems. . . 10
Edge Drain Piping . . . 10
Daylighting the Subbase . . . 11
Separators . . . 11
Chapter 3 – Subgrades
. . . 13Soil Basics for Pavement Construction . . . 13
Moisture Content and Density . . . 13
Soil Water . . . 13
Moisture Equivalent . . . 14
Soil Moisture Suction (Capillary Action). . . 15
Plastic Soils . . . 15
Nonplastic Soils . . . 16
Load Bearing Capacity . . . 17
Volume Stability . . . 19
Classification Systems . . . 19
AASHTO Soil Classification System . . . 20
ASTM (Unified) Soil Classification System . . . 21
Soil Texture Method. . . 24
Subgrade Strength and Working Platform . . . 25
Obtaining Uniform Support . . . 25
Expansive Soils . . . 26
Compaction and Moisture Control of Expansive Soils . . . 27
Chemical Modification . . . 30
Material Selection and Dosage Rates . . . 30
Constructing Modified/Stabilized Subgrades . . . 31
Special Methods for Controlling Expansive Soils . . . 31
Frost Action. . . 33
Frost Heave. . . 33
Frost-Susceptible Soils . . . 35
Spring Subgrade Softening . . . 36
Control of Frost Heave . . . 37
Grade and Water Table Elevation . . . 37
Selective Grading and Mixing . . . 37
Removal of Silt Pockets . . . 37
Compaction and Moisture Control . . . 37
Drainage. . . 38
Protection for Utilities Located in the Subgrade . . . 38
Non-Frost-Susceptible Cover. . . 39
Wet Soils. . . 39
Special Considerations of the Subgrade During Reconstruction Due to Intersection Replacement, Utility Cuts or Inlays . . . 40
Chapter 4 – Subbases
. . . 41Pumping. . . 41
When to Use a Subbase . . . 43
Subbase Types . . . 44
Unstabilized (Granular) Subbases . . . 45
Material Requirements . . . 45
Gradation Control . . . 46
Discussion on Drainage of Unstabilized (Granular) Subbases . . . 47
Quality Control . . . 47
Consolidation. . . 47
Thickness and Compaction. . . 48
Unstabilized (Granular) Subbase Construction . . . 49
Paving Precautions . . . 50 Stabilized Subbases . . . 50 Cement-Stabilized Subbases . . . 52 Cement-Treated Subbases . . . 53 Material Requirements . . . 53 Construction . . . 54
Material Requirements . . . 55
Strength Properties . . . 56
Construction . . . 57
Asphalt-Treated Subbases . . . 58
Design & Material Requirements . . . 58
Construction . . . 59
Stabilized Subbase Precautions . . . 59
Alternative Subbase Materials . . . 60
Recycled Concrete . . . 60
Aggregate Characteristics . . . 61
Precautions . . . 62
Waste Materials. . . 62
Permeable Subbases: Reasons to Avoid Their Use . . . 62
Loss of Support Due to Breakdown of the Aggregate . . . 63
Loss of Support Due to Infiltration of the Subgrade into the Subbase. . . 64
Early Age Cracking Due to Penetration of Mortar from the Concrete Pavement into the Subbase . . . 64
Instability as a Construction Platform . . . 64
Overall Field Performance . . . 65
Cost Effectiveness. . . 65
Special Considerations of the Subbase During Reconstruction Due to Intersection Replacement, Utility Cuts or Inlays . . . 66
Chapter 5 – References
. . . 67Appendix
. . . 73 Glossary . . . 73 AASHTO Standards . . . 81 ASTM Standards . . . 82Index
. . . 83Executive Summary
Analysis of the Federal Highway Administration’s (FHWA’s) Long-Term Pavement Performance (LTPP) data reveals that a pavement’s foundation (base or subbase and subgrade) is one of the most critical design factors in achieving excellent performance for any type of pavement.* For concrete pavements, the design and construction require ments of a roadbed or foundation structure may vary con siderably, de -pending upon subgrade soil type, environmental conditions, and the amount of anticipated heavy traffic. In any case, the primary objective for building a roadbed or foundation for concrete pavement is to obtain a condition of uniform support for the pave-ment that will prevail throughout its service life. Drainage considerations are also important in the proper design and construction of a roadbed or foun-dation for concrete pavement. It is important not to build a supporting layer system that holds water underneath the pavement slabs. This has been a common mistake in the design of concrete pavement
structures, which has led to poor field performance of some concrete pavement sections. It is equally important not to over design the permeability of a subbase layer. Overzealous engineering of a perme-able subbase will most likely lead to a foundation that does not provide the requisite stability for long-term pavement performance. Where stability has been sacrificed for drainage, concrete pavements have performed poorly and have experienced unac-ceptable numbers of faulted joints and cracked slabs
within a relatively short period. Free-draining and
daylighted subbases are the reasonable alterna-tives to rapidly draining permeable subbases with an edge drainage system that often lack sta-bility for long-term perfor mance or cause other performance problems.
In northern or cold climates, the influence of frost and freezing of the roadbed is an important consid-eration. Certain subgrade soils are particularly sus-ceptible to frost action, which raises the foundation and concrete pavement layer(s) vertically during freezing periods (commonly referred to as heaving or frost heaving). Generally, frost heave is limited to areas of freezing climates with silty soils. If the heaving is uniform along a pavement section it is not detri mental, but if heaving is localized, it upsets the unifor mity of support provided to the surface pave-ment. Removing or treating these materials will be necessary to ensure that the pavement performs as expected.
For nearly every pavement design there are many different subbases to choose from (i.e., unstabilized recycled concrete aggregate, cement-treated, lean
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* The use highly open-graded or permeable subbases for concrete pavement is not recommended. This conclusion was reached through experiences with poorly performing pavements built on permeable subbase layers. It is supported by a national performance evaluation study that concluded that these systems do not have a significant influence on pavement performance, positively or negatively (NCHRP 2002). However, the cost of these systems can be quite significant, sometimes as much as twenty-five percent of the section cost com-pared to a more conventional subbase (Cole and Hall 1996). For these reasons, and others described in this publication, the following categories of subbase layers are not recommended: cement-treated permeable subbase, asphalt-treated permeable subbase and unstabi-lized open-graded subbases with a permeability coefficient more than about 350 ft/day (107 m/day) in laboratory tests.
concrete, etc.), as well as the decision of a natural or a treated subgrade. In some cases, as for most clays and some silty soils, it may be most econom-ical and advanta geous to treat the subgrade soil and then to provide a unstabilized (granular) subbase as a construction platform. In the case of a road for a relatively low level of traffic it is likely that a natural subgrade may suffice, as long as it is evaluated to be acceptable as a roadbed. The optimal subbase and subgrade design or selection must balance both cost and performance consid erations. The same combination of subbase and subgrade treatment used for heavily-trafficked highways is likely not necessary for a low-volume roadway, even in the same area and subject to the same climate.
Finally, it is likely that as this document is printed and distributed, some new and emerging technolo-gies are advancing within the grading and paving industries. This guide captures the fundamental parameters, recommendations, and considerations for subgrades and subbases for concrete pavement. Emerging technologies, such as intelligent compac -tion and GPS-guided grading/placing equipment, are likely to become more commonplace in the future. These improvements to existing methods are not a replacement for the necessary consideration of the fundamentals. By the same token, we encourage agencies and contractors to advance their construc-tion methods and improve the quality of their work using advanced technology.
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Executive Overview
For quick reference, key concepts for each chapter are listed as follows and indexed to the tabs on the page edges of this publication.
Key Point
Page
● Roadbed (subgrade and subbase) design is key to long-term performance and
smoothness of concrete pavements.
1
● The pavement structure of a concrete pavement typically consists of a concrete
sur-face and subbase(s) placed upon a prepared subgrade (a “base” is part of an asphalt pavement struc ture, while a subbase is an optional element of a concrete pavement structure).
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Chapter 1. Introduction and Terminology – Page 1
Key Point
Page
● Every foundation for a concrete pavement structure should be free from abrupt
changes in character of the materials (should be uniform), should resist erosion, and be engineered to control subgrade soil expansion and frost heave.
3
● Above all other design concerns, uniformity is of utmost importance.
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● Because of the rigid nature of concrete pavements, loads are distributed over
rela-tively large areas, greatly reducing stresses on the subgrade/subbase; thus, con-crete pavements do not necessarily require exceptionally strong foundation support.
4
● The pavement design engineer should consider all subbase types
(stabi-lized or unstabi(stabi-lized) and available materials (recycled or virgin) for each pavement design; there is no standard recommended subbase for any crete pavement. Subbase selection is the designer’s option, but should con-sider fundamentals and decision factors described in this guide.
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Key Point
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● Soil classification systems such as the AASHTO and/or ASTM (Unified) Soil
Classifi-cation Systems will help the pavement design engineer determine factors such as the California Bearing Ratio (CBR) or modulus of subgrade reaction (k-value), but the engineer must be mindful of the preferred soil classification method for each project because conversion between methods is not intuitive.
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● A minimum CBR of 6 in the top 24 in. (610 mm) of subgrade provides an adequate
working platform for construction, while limiting subgrade rutting under construction traffic to ½ in. (13 mm) or less.
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● Typically, a specified percentage of compaction of 95 percent, according to
AASHTO T99 will provide an adequate working platform for construction equipment and for excellent in-service performance of the subgrade portion of a concrete pave-ment structure.
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● Special attention should be given to expansive and frost-susceptible soils. Expansive
soils can be mitigated by compacting the subgrade at the proper moisture content, selectively grading the subgrade material and/or chemically modifying the sub grade. Frost heave can be mitigated by controlling the grade and water table elevation, selectively grading and mixing the subgrade, removing silt pockets and re fill ing with select borrow materials. It also can be mitigated by covering the existing subgrade with a non-frost -susceptible cover and/or compacting the subgrade at the proper moisture content.
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Key Point
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● Recommended minimum subbase thicknesses are 4 in. (100 mm) for
unsta-bilized subbases, 4 in. (100 mm) for cement-staunsta-bilized subbases (i.e., cement-treated subbases and lean concrete subbases), and 2 in. (50 mm) for asphalt-stabilized subbases.
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● Concrete pavement design thickness is relatively insensitive to support stiffness
(modulus of subgrade reaction), so it is improper engineering to make a subgrade/ subbase stronger or thicker in an attempt to decrease concrete pavement thickness.
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● Free-draining subbases are preferred over permeable subbases.
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● Daylighted subbases are more economical and yield better long-term performance
than edge drain piping.
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Chapter 2. Design Principles – Page 3 (Continued)
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Key Point
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● For pumping of a subbase to occur, several conditions must exist. They are:
■the pavement must have undoweled joints or joints with poor load
transfer,
■water must be present,
■the roadway must have fast moving, heavy loads to deflect the slabs
(trucks, not automobiles),
■and the subgrade must be a fine-grained material or the subbase must
be an erodible material.
Eliminating one or more of these casual factors should mitigate pumping.
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● Pavements that are expected to carry 200 trucks or fewer per day (or less
than 1,000,000 18-kip (80 kN) ESAL’s over the course of the service life of the pavement) do not typically require a subbase to prevent pumping.
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● Unstabilized subbases must have a maximum particle size of no more than
1⁄3the subbase thickness, less than 15 percent passing the No. 200 (75 µm)
sieve, an in-place density of 95 percent according to AASHTO T99, a Plas-ticity Index of 6 or less, a Liquid Limit of 25 or less, a L.A. abrasion resis-tance of 50% or less, and a target permeability of no more than 350 ft/day (107 m/day) in laboratory tests. Of these, limiting the percent of fines passing the No. 200 (75 µm) sieve is of utmost importance to creating a good unstabilized subbase.
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● The higher degree of support offered by a stabilized subbase will not alter
the required concrete pavement slab thickness appreciably, but it will add pumping resistance and increase the overall strength of the pavement struc-ture, spreading loads over larger areas and reducing strains.
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● There is typically no strength requirement for cement-treated subbases
(CTB) because a CTB is best controlled using compaction and/or density requirements. However, when specified, a target compressive strength range of 300 to 800 psi (2.1 to 5.5 MPa) is typical to ensure long-term dura-bility to repeated cycles of wetting and drying or freezing and thawing, while keeping the layer from getting too stiff.
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● Material requirements oftentimes may be relaxed for cement-stabilized
sub-bases (i.e., cement-treated subsub-bases or lean concrete) when compared to unstabilized subbases. For example, granular material used in a cement-treated subbase may have up to 35 percent of particles passing the No. 200 (75 µm) sieve and a Plasticity Index of up to 10.
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Chapter 4. Subbases – Page 41
Key Point
Page
● Strength of a lean concrete subbase should be limited to 1,200 psi (8.3
MPa) or less to keep the subbase from getting too stiff, minimizing curling and warping stresses in pavement slabs. If this strength is exceeded, mea-sures may need to be taken (i.e., scoring joints into the lean concrete sub-base) to mitigate the potential problems.
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● Recycled concrete and other alternative subbase materials should be
considered for inclusion in a subbase for their positive economic and envi-ronmental benefits, as well as resource conservation.
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● Permeable subbases (subbases with a permeability of 350 ft/day (107
m/day) or greater in laboratory tests) have had a problematic history in the field. The reasons include loss of support caused from aggregate break-down, loss of support caused from infiltration of the subgrade into the sub-base, early age cracking caused from penetration of concrete mortar into the subbase voids during paving, instability as a construction platform, cost effectiveness, and various other overall field performance problems. Thus, permeable subbases are no longer recommended for concrete pavement structures. Free-draining subbases (subbases with a permeability between 50 and 150 ft/day (15 and 46 m/day) in laboratory tests) and daylighted subbases are the reasonable alternative to rapidly draining permeable subbases.
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Chapter 4. Subbases – Page 41 (Continued)
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Chapter 1.
Introduction and Terminology
The design and construction of the roadbed for any pave ment structure is key to its long-term perfor-mance and smoothness over time. A roadbed is characterized by the layer(s) that provide the foun-dation for the riding surface. For concrete pavement, the foundation is typically comprised of a subbase layer on top of the subgrade soil. A variety of engi-neered subbase materials and subgrade treatment methods exist for use with concrete pavement. Careful attention to the design and construction of sub grades and subbases is essential to ensure the structural capacity, stability, uniformity, durability, and smoothness of any concrete pave ment over the life of that pavement. Of utmost importance is the uni -formity of the foundation. This bulletin publication discusses each essential factor and provides the necessary background information for the proper selection and application of subbases and the appropriate consideration of subgrade variables for concrete pavements used for streets, roads, and highways.
Because the terminology for engineered roadbeds is unique and sometimes unfamiliar to pavement design engineers, an extensive glossary of terms is included as an Appendix of this publication. (Refer to the tabs at the edge of this publication for quick reference.) Thus, this section is not in tended to be a comprehensive glossary, but a means of distin guish -ing between foundation components for concrete and asphalt pavement structures. The key terms necessary for discerning be tween concrete and asphalt pavement structures are:
• Pavement Structure — The combination of
asphalt/concrete surface course(s) and base/subbase course(s) placed on a prepared subgrade to support the traffic load.
• Base — A layer within an asphalt pavement
struc-ture; usually a granular or stabilized material, either previously placed and hardened or freshly placed, on which the pavement surface is placed in a later operation.
• Base Course — The layer(s) of hot mix asphalt
immediately below the surface course, generally consisting of less asphalt and larger aggregates than the surface course. Also known as binder course (AI 2007).
• Subbase — The layer(s) of select or engineered
material of planned thickness placed between the subgrade and a concrete pavement that serve one or more functions such as preventing pumping, distributing loads, providing drainage, minimizing frost action, or facilitating pavement construction. Common subbase types include unstabilized (granular) subbase, cement-treated subbase, lean concrete (econocrete) subbase and apshalt-treated subbase.
• Subgrade — The natural ground, graded and com
-pacted, on which a pavement structure is built. In practice, subbase layers are commonly referred to as base courses. Strictly speaking, however, a base course is a layer of material beneath an asphalt sur-face; thus, base courses exist only under asphalt pavements and subbases exist only under concrete pave ments. The pressures imposed on a base
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course underneath an asphalt pavement are dramat-ically different than those imposed on a subbase beneath a concrete pavement. Because of this dif-ference, material quality requirements for a subbase
may be relaxed in comparison to what is required for a base. The distinction in terminology (base versus subbase) recognizes these basic differences. This publication discusses concrete pavement subbases, and the reader is encouraged to adopt this term into their common or local terminology. Figure 1 illus-trates concrete and asphalt pavement structures.
Asphalt Surface Course
Subgrade
Asphalt Pavement Structure
Base C
ourses
Subgrade
Concrete Pavement Structure
Concrete Pavement
Subbase 1
Subbase 2
Figure 1. Cross-sectional illustration of the relative difference in design terminology and layout between asphalt and concrete pavement structures.
Chapter 2.
Design Principles
Understanding the basic premise and principle of foundation design for concrete pavement requires a knowledge of how concrete slabs transfer loads from vehicles to the subgrade. Compared to asphalt pavements, concrete pavements spread a given load over a larger area of the roadbed or foundation which, in-turn, reduces the pressure on the support layer materials and subgrade. The importance is that the foundation strength is not as important to the performance of concrete as it is to asphalt pave-ment, even when considering pavements for heavy loads.
Although subbase and subgrade strength are impor-tant factors in pavement design, other foundation properties besides strength need to be considered in the design of a foundation for concrete pavement. Every foundation for use in a concrete pavement structure should provide the following characteristics: • Uniformity; no abrupt changes in character of the
materials (i.e., weak spots or stiff spots). • Control of expansive subgrade materials to
ensure uniform support through wet and dry sea-sons.
• Resistance to frost heave during winter and cold temperatures.
• Resistance to erosion by slabs that deflect under heavy loads.
Of these characteristics, uniform support is of utmost importance. Providing uniformity is also one of the largest challenges in the design and construction of any pavement structure. Because every foundation
design starts with the in-situ natural soils, the chal-lenge always begins with the subgrade. In practical terms, the subgrade must, at least, provide a stable working platform for constructing the subsequent layers of the pavement structure.
The potential for frost heaving and/or shrinkage and swelling of subgrade materials must be assessed by the engineer during the design phase. The methods available to address expansive subgrade materials are selective grading and/or chemical modification (commonly referred to as soil stabilization) of the in-situ soils. Both of these subgrade conditions (e.g. frost heave and shrink/swell) should be considered separately from providing pavement support, but are inherently part of the primary goal of providing a uni-form foundation. In other words, even though a sub-grade can be compacted and prepared to provide adequate support for construction activities and future traffic loading, it may be a poor foundation for a concrete pavement if the subgrade is prone to volume change from swelling, shrinking, or heaving. Therefore, the expansive potential of the subgrade must be evaluated and controlled.
Preparation of the subgrade includes:
• Compacting soils at moisture contents and densi-ties that will ensure uniform and stable pavement support.
• Whenever possible, setting the profile gradeline at an elevation that will allow adequate depth in the side ditches to protect the pavement structure from the water table.
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• Improving expansive or weak soils by treatment with portland cement, fly ash, cement kiln dust (CKD), lime, or alternatively, importing better soils. • Cross-hauling and mixing of soils to achieve
uni-form conditions in areas where there are abrupt horizontal changes in soil types.
• Using selective grading in cut-and-fill areas to place the better soils closer to the top of the final subgrade elevation.
• Fine grading the top of the subgrade to meet specified grade tolerances in the specifications and for thickness control of the subbase and/or the concrete pavement.
Perfect subgrade materials—those that would eco-nomically meet all design criteria—are rarely encountered in nature. This is particularly true of materials that would be used in heavily trafficked pavement. For this reason, a subbase layer provides an added measure of assurance that both uniform support and a non-erodible layer are provided for the concrete pavement slabs. Subbases consist of engi-neered materials or materials that are produced and controlled to a specification. Most commonly used subbases fall into one of the following categories: • Unstabilized (granular) subbases.
• Stabilized subbases, which include:
■ cement-stabilized subbases (cement-treated
subbases or lean concrete subbases, both of which may include fly ash and/or slag) and
■ asphalt-treated subbases.
For light traffic pavements, such as residential streets, secondary roads, parking lots, and light-duty airports, a subbase may not be required if proper subgrade preparation techniques will minimize shrink, swell, and/or heave potential, provide an ade-quate construction platform and provide adeade-quate pavement support.
When the use of a subbase is considered appro-priate, the best results are obtained by:
• Selecting subbase materials and combinations of layers that adequately prevent pumping of sub-grade soils for the life of the pavement.
• Specifying gradation controls that will ensure a reasonably constant subbase gradation for indi-vidual projects.
• Specifying a minimum density of 95 percent of AASHTO T99 (ASTM D698) for unstabilized subbases.
• Specifying stabilized subbase material require-ments (cement-treated, lean concrete, or asphalt-treated) that consider the delicate balance
between the requirement of uniform support and the risk of cracks associated with high strength subbases due to loading of unsupported edges (caused by curling and warping).
• Designing the width of the subbase to accommo-date the paving equipment. The subbase should extend beyond the width of the pavement by at least 3 ft (1 m) on either side to provide a stable all weather working platform for the paving equip-ment or fixed side forms. This additional width of subbase is a critical feature to help ensure smoother pavements. Secondary benefits over the life of the pavement include improved load transfer at the edge of the concrete slab.
• Specifying a minimum subbase thickness of 4 in. (100 mm) for unstabilized (granular) subbases, 4 in. (100 mm) for cement-stabilized subbases and 2 in. (50 mm) for asphalt-treated subbases.
UNIFORM SUPPORT
Paving concrete typically has a 28-day flexural strength ranging from 550 to 750 psi (3.8 to 5.2 MPa), or greater, and a modulus of elasticity ranging from 4 to 6 million psi (28,000 to 41,000 MPa), helping provide a high degree of rigidity. This rigidity enables concrete pavements to distribute loads over large areas of the supporting layers, as shown in Figure 2. As a result, pressures on the underlying layer(s) are very low and deflections are relatively small. Concrete pavements, therefore, do not neces-sarily require exceptionally strong foundation
much higher pressure transmitted by wheel loads through each layer and ultimately to the subgrade. The importance of the principle of uniform subgrade support is best explained by anomalies in pavement performance from the field. Performance surveys have been conducted over many miles of old con-crete pavements that were constructed without proper subgrade compaction control and without subbases. Where the subgrade was naturally uni-form, many of these old pavements are still in excel-lent condition. Distress is limited to cut-fill transitions and other locations where there are abrupt changes in subgrade materials and moisture conditions. Sur-veys show that low-strength soils where construction methods provided reasonably uniform support per-form better than stronger soils lacking uniper-formity (ACPA 1995).
Chapter 2 — Design Principles
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7,000lb (3,200 kg) 16 ft (4.9 m) Pressure Only Pressure Only 3 psi (0.02 MPa) 3 psi (0.02 MPa) 8 in. (200 mm) Concrete SlabFigure 2. The rigidity of concrete helps a concrete pavement distribute wheel loads over large areas, keeping
sub-base/subgrade pressures low. A 12,000 lb (5,400 kg) load is placed on a 12 in. (380 mm) plate.
This yields a pressure of 106 psi (0.73 MPa) on the pavement surface and the resultant subgrade pressures listed above.
Loading Position
1. Slab Interior 2. Outside Edge 3. Outside Corner 4. Transverse Joint Edge
Maximum Subgrade Pressure psi MPa 0.02 0.04 0.05 0.03 3 6 7 4 Clay Loam Subgrade 8 in. (200 mm) Concrete Slab Subbase 1 2 3 4 Doweled Joint
Figure 3. Subgrade pressures for a 12,000 lb (5,400 kg) load applied at several positions on a slab.
Childs and Kapernick (1958) showed that heavier loads are distributed over large areas of the sub-grade and, thus, do not cause high subsub-grade pres-sures. Figure 3 gives test conditions and subgrade pressures for a 12,000 lb (5,400 kg) load. The applied pressure of 106 psi (0.73 MPa) was reduced to subgrade pressures of only 3 to 7 psi (0.02 to 0.05 MPa) because the applied load is distributed over more than 20 ft (6 m). Other studies (Childs, Colley, Kapernick 1957; Childs and Nussbaum 1962; Childs and Kapernick 1963) confirm that the sub-grade pressures below a concrete pavement struc-ture are quite low and, in fact, considerably less than the bearing strengths of almost all subgrades. For a concrete pavement structure, it is extremely im portant that the support be reasonably uniform with no abrupt changes or isolated weak or stiff spots in the character of the foundation. This is in contrast to the principle of design for asphalt pavements, where successively stronger base layers are re -quired closer to the surface layer to distribute the
INFLUENCE OF FOUNDATION
STRENGTH ON CONCRETE
PAVEMENT THICKNESS
Although subbases are used to increased composite support strength and protect the subgrade, it is the subgrade that must ultimately bear the load, making it the starting point for support characterization and design. As mentioned, the primary requirement of the subgrade beneath a concrete pavement struc-ture is that it be uniform. This is the funda mental reason for specifications on subgrade compaction. While a uniform, good-quality, and properly-com-pacted subgrade will improve the performance of the pavement, it is not necessarily true that a stronger subgrade will do the same; most of the structural capacity of a concrete pavement structure is sup-plied by the concrete slab and not by the foundation (subgrade and/or subbase).
The strength of the foundation for a concrete pave-ment structure is often quantified as the modulus of subgrade reaction (k-value). The modulus of sub-grade reaction is determined by the plate load test (AASHTO T222 or ASTM D1196). The plate load test models the subgrade as a bed of springs, with the k-value being analogous to the spring constant; in fact, k is sometimes referred to as the subgrade “spring constant.” The test involves placing a 30 in. (762 mm) diameter plate on the subgrade and load -ing it with a very heavy load. The plate distributes the load to the subgrade via the pressure on the face of the plate. The k-value is found by dividing the plate pressure by the plate deflection under the load. The units for k-value are psi/in. (MPa/m).
An exact k-value of the subgrade is not typically required; a measured subgrade k-value is heavily dependent on the season, moisture conditions, loca-tion, etc. Furthermore, when a subbase system is used, there can be a significant increase in the com-posite k-value and an exact value of the k-value of the subgrade is of even less concern. The composite k-value may be measured by a repetitive static plate load test (AASHTO T221 or ASTM D1195) for use in
design or evaluation of components of the concrete pavement structure. This test, which is widely used in Europe, is a modification of the standard plate load test used on subgrades (AASHTO T222 or ASTM D1196). It includes repeated loading and bearing plate diameters down to 6 in. (150 mm), to more accurately model a vehicular load.
The magnitude of the increase in k-value from the inclusion of a subbase in the design of the pave -ment system depends on the subbase material and whether the subbase is treated or untreated. Normal variations from an estimated subgrade or composite k-value will not appreciably affect pavement thick-ness in typical k-value ranges, as shown in Figure 4. Note that it is not economical to use an
over-designed sub base system for the sole purpose of increasing the composite k-value; increasing the slab thickness, concrete strength, edge support and many other variables often proves to be more economical. Figure 4 shows an increase in the k-value from 100 psi/in. (27 MPa/m) to 500 psi/in. (135 MPa/m), which will only decrease the required concrete slab thickness by about 20 percent.
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k-value [psi/in. (MPa/m)]
C o ncr e te P a v ement T hick n ess [in. (mm)] 100 (27) 150 (41) 200 (54) 250 (68) 300 (81) 350 (95) 400 (109 ) 450 (122) 500 (136) 12 (300) 10 (250) 8 (200) 6 (150) 4 (100) 2 (51) 0 (0) Major Arterial Residential Street
Figure 4. Sensitivity of k-value for a residential street and a major arterial. Assumptions for the residential street include: 12 ft (3.7 m) joint spacing, no dowel bars, 20 year design life, ADTT of 3, and a flexural strength of 600 psi (4.1 MPa). Assumptions for the major arterial include: 15 ft (4.6 m) joint spacing, 1.25 in. (32 mm) diameter dowel bars, 20 year design life, ADTT of 10,000, and a flexural strength of 600 psi (4.1 MPa).
INFLUENCE OF FOUNDATION
STIFFNESS ON STRESSES AND
STRAINS IN CONCRETE
PAVEMENT SLABS
Although concrete pavement is commonly referred to as ‘rigid,’ because of its relatively high modulus of elasticity when compared to asphalt pavement (commonly referred to as ‘flexible’), the modulus of elasticity of concrete is not so high that a concrete pave ment does not deflect under a heavy load. In fact, once a crack initiates in a concrete pavement (top-down at a corner or mid-slab, bottom up at the mid-slab, etc.), the zone immediately around the crack tip is more appropriately modeled as pseudo-elastic than brittle as the term rigid might suggest. The subsequent opening and propagation of the crack tip is the origin of concrete pavement fatigue, which can result in distresses such as mid-slab transverse cracks.
When a concrete pavement is placed either on the subgrade or on any number of subbase layers, the properties of these foundation layers will directly influence the stresses and strains of the concrete slabs and, in turn, have some bearing on the long-term performance of the system. The most often utilized common material property used in quanti-fying this inter action between the foundation and the concrete slab is the modulus of elasticity, often mea-sured indirectly by the compressive strength in
stabi-lized subbases. Figure 5 illustrates how the concrete pave ment, composite subbase layers and composite subgrade might be modeled in a modern design analysis, showing the combining of support modulus (stiffness) in layers under the concrete pavement. Counter to intuition, the stronger and stiffer the foun -dation becomes, the more problematic it may be for concrete pavement performance. If the concrete slab is in full contact with the foundation, a stiffer support system will reduce deflec tions and, thus, stresses under heavy loads. If a concrete pavement could be constructed on a perfectly rigid foundation (infinite modulus of elasticity) and remain perfectly planar, there would be zero deflection and zero flexural stress, the primary mode of fatigue failure in con-crete pavements. Stiffer support systems, however, will increase deflections and stresses under envi -ronmental loading (thermal curling and moisture warping); if a concrete pavement is constructed on a very rigid foundation, the foundation is not capable of conforming to the shape of the slab so support from the foundation might be lost upon environ-mental loading. The converse is true for a concrete pavement built upon a very flexible foun dation, with higher stresses resulting from applied loads due to free deformation and lower stresses under environ-mental loading because the foundation conforms to the slab shape. These two extremes of foundation
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Natural Subgrade
Compacted Subgrade
Bedrock
Econcrete Esubbase Effective k-value obtained through backcalculationConcrete Pavement
Concrete Pavement
Composite Subbases
Subbase 2
Subbase 1
support are illustrated in Figure 6. A balance of strength and flexibility in the foundation system is necessary for excellent long-term pavement perfor-mance, placing the best foundation support condi-tions between Case 1 and Case 2.
Higher curling stresses have a more damaging impact when the concrete is relatively young, when the slab has not yet developed the strength and fracture toughness necessary to resist cracking. Strength is important to prevent the initiation of a crack and fracture toughness is important to prevent propagation of a crack. If the stiffness of a stabilized subbase becomes too great, not only will the curling stresses in the pavement slabs increase, but the probability of reflective cracks from a stabilized sub-base will also increase (assuming drying shrinkage cracking has occurred in the subbase). Also, the thicker a subbase layer is constructed, the greater the increase in support stiffness.
The pavement design engineer must recognize that subbase thickness and stiffness (by way of compressive strength) are important on a concrete pavement foundation system. Recommended minimum subbase thicknesses are 4 in. (100 mm) for unstabilized subbases, 4 in. (100 mm) for cement-stabilized subbases and 2 in. (50 mm) for asphalt-treated subbases. Unstabilized subbases and cement-treated subbases are best controlled using compaction and/or density requirements. Cement-treated subbases should be in a strength target range of 300 to 800 psi (2.1 to 5.5 MPa) (PCA 2006), while lean concrete subbases require a maximum strength limit of 1,200 psi (8.3 MPa). Methods to mit-igate prob lems due to excessive strength are dis-cussed through out this publication.
PAVEMENT SYSTEM DRAINAGE
It is important for the reader not to overlook drainage as unimportant in concrete pavement design. Quite the contrary is true. Drainage, however, must be put into the proper perspective as just one element of many needed to optimize performance of a concrete pavement structure. Uniform support is the primary driver of good performance and the most important fundamental in engineering and select ing a sub-grade and subbase combination for a concrete pave-ment.
In many respects, drainage should be addressed in preparing a subgrade and shaping the roadway tem-plate with ditches and adequate horizontal and ver-tical sloping; however, consideration of drainage in subbase layers is also important.
Sources of Moisture in a Pavement
Structure
The sources of moisture to a pavement structure are shown in Figure 7. The significance of the influence of moisture on the performance of pavements cannot be ignored. However, an engineer must also recognize that even though there may be some con-trols possible, drainage systems such as subsurface edge drains, edge ditches, and culverts are never an absolute control for preventing moisture from gaining
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Ematerial = Applied LoadDue to the perfectly rigid foundation, no deflections or flexural stresses develop.
During environmental loading, the foundation does not conform to the slab and support is lost.
Ematerial =
Loss of support results in high stresses in the concrete slab upon loading.
Due to the lack of support, the concrete slab is free to deflect and high flexural stresses develop.
Applied Load
Ematerial = 1 psi (0.007 MPa)
During environmental loading, the foundation conforms to slab, maintaining support.
Ematerial = 1 psi (0.007 MPa) Case 1: The foundation is perfectly rigid.
Case 2: The foundation is very flexible.
Figure 6. Illustration of the effects of foundation support on applied and environmental loads in a concrete pavement system. The best foundation support condition for a concrete pavement is somewhere between Case 1 and Case 2.
access to the pavement. Instead, drainage systems are tools for minimizing moisture variations in the confines of a pavement structure to within reasonable limits (maintain equilibrium). Extremes in mois -ture variation (non-uniformity) contribute more to pavement distresses and problems than the pres-ence of moisture alone.
Capillarity is the action by which a liquid (water) rises or wicks in a channel above the horizontal plane of the supply of free water (water table). The number and size of the channels in a soil determine its pore size distribution and thus its capillarity. This soil property is measured as the dis tance (ranging from zero to 30 ft (9.1 m) or more) moisture will rise above the water table by this action. Moisture in clay soils may be raised by capillarity for vertical distances as great as 30 ft (9.1 m), considered by highway en -gineers to be a “high capillarity” material. However, a long period of time is often required for water to rise the maximum possible dis tance in clay soils because the channels are very small and frequently interrupted. Silts also have high capillarity, but maxi -mum capillary rise occurs in even a longer period of time than for clayey soils because the pores in a silty soil are sufficiently large to greatly reduce the capillary action. The capillary rise in gravels and coarse sands varies from zero to a maximum of a
few inches because the pores are large enough to eliminate almost all capillary action.
The water table beneath a pavement will rise and fall due to seasonal and annual differences in pre ci -pi tation (i.e., number 5 in Figure 7 is dependent on number 1). A higher water table will result in a greater driving force for capillary suction and vapor move-ment near the subgrade (i.e., numbers 2 and 4 are dependent on number 5). Thus, the design of the pavement structure must assume the highest water table expected during the life of the pavement, be -cause that is when the subgrade and subbase will contain the moist moisture and be the weakest. The highest water table during the life of a pavement should be ex pected around the most major precipi-tation event, making runoff from areas of higher ele-vation most detrimental to a local water table (so number 3 is dependent on number 1).
In an effort to minimize moisture levels in the pave-ment structure, roadway engineers often concentrate on the easiest sources of moisture to isolate, which are numbers 1, 3, and 5. Often, highways are elevated with respect to their surroundings, a configur -ation that forces water to run downhill to ditches (mitigating numbers 1 and 3) while, at the same time, increasing the distance between the pave ment structure and the water table (mitigating number 5 and, in turn, minimizing numbers 2 and 4). Since an elevated roadway is typically not possible for street or road applications, edge drains and sewers are used to collect any surface runoff (mitigating num-bers 1, 3, and 5).
Free-Draining Subbases
Free-draining subbases are preferred over perme-able subbases because of their more durperme-able, more stable nature (Figure 8). The recommended target permeability (k) for free-draining subbase materials is between 50 and 150 ft/day (15 and 46 m/day) in laboratory tests. Materials providing as much as 350 ft/day (107 m/day) in laboratory tests may also provide adequate long-term stability for a pavement foundation.
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1. Precipitation and entry from the pavement edge 2. Capillary suction from the water table
3. Drainage from natural high ground 4. Vapor movement through the soil 5. Water table rise in elevation
Concrete Pavement Treated Subgrade Water Table 3 1 1 5 2 4
Older recommendations for unstabilized permeable subbases suggest a target permeability in the range of 500 to 3,000 ft/day (150 to 315 m/day) in labora-tory tests (FHWA 1992). However, material with this high degree of permeability (above approximately 350 ft/day (107 m/day)) also has a high degree of void space, which decreases stability. Field reports from contractors indicate difficulty in constructing pavements on these open-graded materials. Trucks, paving machines, and other heavy equipment dis-place unstabilized materials that are open in their gradation (consists of mostly one aggregate size). Con tractors have used the description “it’s like paving on marbles” to describe paving on a perme-able subbase.
Though free-draining subbases drain slower than permeable subbases (because of the increased fines content) they still drain more quickly than con-ventional, dense-graded subbases. Stability is enhanced by the use of aggregate that is angular and does not degrade under repeated loading. Recycled concrete aggregate (either from an existing concrete pavement or another source) produces good results in free-draining subbases; however, it should be noted that recycled aggregate subbase has a lower permeability, strength, and resistance to particle degradation than limestone or gravel subbases.
Edge Drainage Systems
An edge drainage system can consist of a collector pipe and outlet system with redundant outlets, or a daylighted subbase system where the subbase ex -tends and carries water to the side ditches. The common application for edge drainage systems is for high volume roadway or highway applications, such as major state roads and interstates. Even then, their use is not always required or suggested. The use of edge drainage systems for low volume applications such as rural roads, county roads, etc., is not suggested. These types of pavements will provide excellent service with fundamental roadbed considerations, such as appropriate ditch and ele -vation design. Additionally, the loading on these pavements is likely to be such that pumping is not a concern. In these situations, a dense-graded unsta-bilized (granular) subbase or construction on appro-priately prepared subgrade will suffice.
Edge Drain Piping
Where edge drains are used, the hydraulic capacity of longitudinal edge drains and outlet laterals must be high enough to drain the free water within the pavement structure within 2 hours of rain cessation (FHWA 1990). The drainage pipes typically consist of a 4 to 6 in. (100 to 150 mm) diameter flexible, cor-rugated polyethylene tubing (perforated) meeting AASHTO M252. Rigid PVC pipe (slotted) meeting AASHTO M278 – PC50 has also been used, but it is considerably more expensive. Trenches are back-filled with highly permeable material to easily draw moisture from the subbase. A filter fabric (geotextile) lines the trench to prevent the fine particles from intruding into the trench area; the filter fabric is extended across the pavement section to prevent fine particles in the subgrade from intruding into the free-draining subbase. The recommended detail for the filter fabric liner is found in Figure 9.
Lateral outlet pipes are made from rigid PVC or metal. Rigid pipe provides more protection against crushing due to con struction or maintenance opera-tions. Although the spacing between outlets has been as much as 300 to 500 ft (90 to 150 m) in
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Figure 8. Free-draining, unstabilized subbase with enough fines to be stable during construction but still provide permeability of about 200 ft/day (60 m/day) in laboratory tests. Note that the truck tires are not causing excessive rutting or displace-ment of the subbase material.
the periodic maintenance that is required for a pipe drain system. Furthermore, studies found that flexible pavements sections with daylighted bases (with -out edge drains) performed as well as (or better than) any other flexible pavement section (NCHRP 2002). Similar performance should be expected with concrete pavements. The recommended details for a daylighted subbase are shown in Figure 10.
Separators
Separators are geotextile fabrics or filter layers that prevent the migration of fines from the subgrade into the free-draining subbase. Geotextile fabrics are commonly used (and strongly suggested) directly below a free-draining subbase layer to prevent fines from infiltrating and plugging the subbase.
Some agencies also place a filter layer (4 to 6 in. (100 to 150 mm) thick layer of dense-grade unstabi-lized granular material) below a drainable subbase. This is not con sidered a necessity when a free-draining subbase material is employed in the design. Where used, the filter layer serves as a construction platform and as a barrier to prevent water from entering the subgrade as it flows through the sub-base to the ditch or edge drain piping.
The following criteria for filter layers are recom-mended. It will be necessary to evaluate both the filter layer/subgrade and the subbase/filter layer interfaces (FHWA 1990, US ACoE 1941):
Subgrade Concrete Shoulder Free-draining Subbase Concrete Pavement Drainable Material Reaches Daylight
Figure 10. Detail for a daylighted subbase. Note that there is no filter fabric (geotextile) or filter layer as there is for an edge drain system. Instead, it is accepted that some local clogging of the permeable layer will take place, but overall drainage will not be lost since the entire depth of the layer is exposed for the entire length of the pavement.
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practice, a maximum of 250 ft (75 m) is preferred to ensure proper drainage. Outlets should also be placed at the bottom of all vertical curves. The pipes should be placed on a 3 percent grade with the outlet at least 6 in. (150 mm) above the 10 year design flow in the ditch. Concrete headwalls are important to protect pipe outlets. Outlets should be equipped with rodent screens.
For crowned pavements, edge drains are installed along both the inner and outer pavement edge. This shortens the drain age path and reduces the time for the subbase to drain. However, for pavement lanes built as an uncrowned section, only one edge drain is installed, at the low side, which is considerably less expensive.
It is important to place the longitudinal edge drain outside of the paver trackline or any location that is expected to receive loads by heavy construction equipment. A minimum offset distance of 3 ft (1 m) is recommended whenever possible.
Daylighting the Subbase
Though often disregarded in the past due to the mindset that overgrowth along the ditch line would clog the system, daylighting a subbase directly into the side ditches may yield better long-term perfor-mance than edge drains, because it does not rely on
Subgrade Geotextile Collector Pipe Concrete Shoulder Free-draining Subbase Concrete Pavement
Minimum offset distance of 3 ft (1 m) Separator Layer (Geotextile)
Figure 9. Detail for edge drain piping. Note that the filter fabric (geo textile) does not completely surround the trench, which prevents the fabric from being clogged by leachates or other fine particles carried by water flowing through the sub-base, and the drain is offset at least 3 ft (1 m) from the edge of paving whenever possible, which protects it from construc-tion traffic.
1. The 15 percent size (D15) of subbase should not be more than 5 times larger than the 85 percent
size (D85) of the filter.
2. The 50 percent size (D50) of subbase should not
be more than 25 times larger than the 50 percent
size (D50) of the filter.
3. The 15 percent size (D15) of the filter should not
be more than 5 times larger than the 85 percent
size (D85) of the subgrade soil.
4. The 50 percent size (D50) of the filter should not
be more than 25 times larger than the 50 percent
size (D50) of the subgrade soil.
Note:The Dxsize means that x percent of the
parti-cles are smaller than this size.
Filter material should not be placed in a manner to obstruct drainage through the subbase or edge piping. The 85 percent size of the subbase should
be at least 11⁄
2to 2 times the size of the slotted pipe
openings.
Various filter design criteria for both aggregate and fabric are available elsewhere (FHWA 1990, US ACoE 1991).
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Chapter 3.
Subgrades
SOIL BASICS FOR PAVEMENT
CONSTRUCTION
Note:This section, Soil Basics for Pavement Con-struction, is taken predominately from the Portland Cement Association’s EB007 “PCA Soil Primer.” Soil forms when rock, marine shell, coral, etc. breaks into smaller and smaller size particles through the processes of abrasion and/or fracturing (physical weathering). Events that contribute to or accelerate this break-down process include wind weathering, erosion, freezing, rock impact, root growth, wetting and drying, heating and cooling, glacial action, and human factors.
Of far greater significance for fine-grained soils, although less intuitive than the physical breakdown process, are modifications by chemical processes (chemical weathering), plant and animal additions, and man’s impact as the soils are transported by flowing water or are subject to moist-to-wet condi-tions in place.
Recognition of these soil forming processes (break-down and modification) is valuable to both the pre-liminary site surveys and the extension of limited subgrade sampling information across a project. Near mountain or upland sources, soils will be coarser and more closely related to the source rocks; downstream or at lower elevations, soils will be fine grained, greatly modified and subject to sorting processes (i.e., wind and water transport). Also, the break-down process will apply more
directly in arctic areas, whereas chemical modifica-tion of soils will be greatest in tropical areas.
Regardless of the method of formation or the source of the soil, long-term subgrade performance
depends heavily on three interdependent factors: • Moisture content and density.
• Load bearing capacity. • Volume stability.
The following sections provide an overview of test methods used to quantify the previously mentioned performance factors. Knowledge of these properties and their interdependence is necessary to under-stand the classification systems presented at the end of this section.
Moisture Content and Density
As mentioned, uniformity of support is of utmost con-cern to a pavement engineer. Also of interest is soil strength. Because soil consists of solid particles, water and air, the moisture condition and, to a lesser degree, the density or unit weight are also of con-cern because they directly influence strength. This section describes various properties relating to mois-ture and density of soils.
Soil Water
A soil mass is a porous material containing solid par-ticles interspersed with pores or voids. These voids may be filled with air, with water or with both air and water. There are several terms used to define the
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relative amounts of soil, air, and water in a soil mass:
• Density — The weight of a unit volume of soil. It
may be expressed either as a wet density
(including both soil and water) or as a dry density (soil only).
• Porosity — The ratio of the volume of voids to the
total volume of the mass regardless of the amount of air or water contained in the voids. Porosity is typically expressed as a percentage.
• Void Ratio — The ratio of the volume of voids to
the volume of soil particles. The porosity and void ratio of a soil depend upon the degree of com-paction or consolidation. Therefore, for a particular soil in different conditions, the porosity and void ratio will vary and can be used to judge relative stability and load carrying capacity with these fac-tors increasing as porosity and void ratio
decrease.
• Degree of Saturation — The ratio of the volume of
water to the volume of voids, usually expressed as a percentage.
The moisture or water content of a soil is normally expressed as a percentage of the oven-dry weight of the soil. It is determined by first taking the difference in weights between a moist soil sample and the same sample dried in an oven at 230 deg F (110 deg C) until it reaches a constant weight. This differ-ence divided by the oven-dry soil weight (expressed as a percentage) is the moisture content. AASHTO T265 or ASTM D2216 describe this test method. In common usage, the terms “moisture content” and “water content” are synonymous.
The moisture or water that makes up the measur-able difference between the in-situ moisture state and the oven-dried state is of three different types:
1.Gravitational Water — Water free to move under
the influence of gravity. This is the water that will drain from a soil. For in-situ soils it is water at and below the ground water table and is often termed “groundwater.” Groundwater is unbound or “free” water.
2.Capillary Water — Water held in the soil pores or
“capillaries” by “capillary action.” This is the result of an attraction between fluids and solid surfaces, which, because of stronger attraction to water than to air, results in the upward curving of a meniscus at the water’s edge and to actual rising of water in a narrow tube. Water pressure is zero at the groundwater level or phreatic surface; it is under pressure below this surface and in tension above. Note that capillary water cannot exist directly in the presence of gravitational water. Effects of gravity on a mass of water result in pressure or compression from the water weight. This overrides the tension and relieves the capil-lary attractions. Capilcapil-lary water is not generally considered to be “free” water since it is, at least weakly, bound by the surface tension action. However, because it is not strongly bound to soil particles directly, it has sometimes been des -cribed as free water in older and especially in agriculturally-oriented soil references.
3.Hygroscopic Water — Moisture retained by soil
after gravitational and capillary moisture are removed. It is held by each soil grain in the form of a very thin film adsorbed on the surface by molecular attractions involving both physical and chemical affinity. This film is in equilibrium with the moisture content of the air and increases or decreases with changes in humidity; it can be described as the water associated with the air-dry moisture content.
■Moisture Equivalent
Both capillary water and hygroscopic water are, to a degree, “bound” and represent a capacity for the soil to hold water against forces tending to remove it. Measures of this “water-holding capacity” are the “moisture equivalent” moisture contents. Low values are associated with coarse-grained soils, which are not moisture sensitive and are highly permeable. High values are associated with plastic clays, which are very moisture sensitive and are of low perme-ability. The tests used to quantify the moisture equiv-alent are:
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• Field Moisture Equivalent — The field moisture equivalent (FME) is the minimum moisture content at which a smooth surface of soil will absorb no more water in 30 seconds when the water is added in individual drops. It shows the moisture content required to fill all the pores in sands, when the capillarity of cohesionless expansive soils is completely satisfied and when cohesive soils approach saturation. This test is no longer common and both standard procedures used to conduct it, AASHTO T93 and ASTM D426, are discontinued.
• Centrifuge Moisture Equivalent — The centrifuge
moisture equivalent (CME) is the moisture content of a soil after a saturated sample is centrifuged for one hour under a force equal to 1,000 times the force of gravity. This test, ASTM D425, is used to assist in structural classification of soils. Low values, such as 12 or less, indicate permeable sands and silts; high values, such as 25, indicate impermeable clays. High values indicate soils of high capillarity, and low values indicate soils of low capillarity.
When both the FME and CME are more than 30 and the FME is greater than the CME, the soil probably expands upon release of load and is classified as elastic.
■Soil Moisture Suction (Capillary Action)
FME and CME have origins in agricultural soil tech-nology, but they found early applications in relation to highway subgrade assessment and right-of-way soil surveys. They continue in some use, but the technology concerned with subgrade moisture-strength in place is now more focused on “soil mois-ture suction.” This is the moismois-ture tension associated with capillarity, and thus, it is often called “capillarity” or “capillary action.”
Water in soil above the water table has a pressure less than atmospheric. It rises above the water table because of the surface tension (capillary forces) and adsorption forces by which the water is bound or held in the soil.
For a soil with measurable soil moisture suction, cap-illarity ranges from zero at saturation to quite large values when the soil is relatively dry. Thus, soil mois-ture suction is dependent not only of the overall driving force of the soil but also the current moisture state. The suction can be expressed in units of (negative) pressure. Relation between the suction and moisture content is very dependent on the soil type. A test standard for measurement of soil suction is pre-sented as AASHTO T273 and ASTM D3152.
■Plastic Soils
Most soils include a fine fraction of silt or clay, or a combination of the two. The consistency of these soils can range from a dry, solid state to a wet, liquid state with the addition of water. Introducing water into a matrix of soil particles, air and water allows empty pore space (space currently occupied by air) to fill with water. Eventually, all of the empty pores will be occupied by water and the addition of any more water will cause the system to expand. If the addition of water occurs in small enough steps, the consistency of silts and clays can be seen passing from solid to semisolid to plastic and to liquid, as illustrated in Figure 11.
The shrinkage limit (SL) separates solid from semi-solid, the plastic limit (PL) separates semisolid from plastic state and the liquid limit (LL) separates plastic from liquid state. The plasticity index (PI) is the width of the plastic state (LL minus PL), expressed in terms of moisture content. The PI is an important indicator of the plastic behavior a soil will exhibit; a low PI is indicative of a very moisture-sensitive soil. Standard procedures have been developed so that consistent determinations to establish the dividing limits can be made by anyone employing these pro-cedures. Since it is the more plastic or finer soils that reflect this pattern of response to moisture variation, the standard tests are performed on the portion of a soil that will pass a No. 40 (425 µm) mesh sieve.
Chapter 3 — Subgrades